Polyamines and hypusination are important for Clostridioides difficile toxin B (TcdB)-mediated activation of group 3 innate lymphocytes (ILC3s)

Clostridioides difficile is the most common cause of nosocomial gastrointestinal tract bacterial infections. We lack fully effective reliable treatments for this pathogen, and there is a critical need to better understand how C. difficile interacts with our immune system. Group 3 innate lymphocytes (ILC3s) are rare immune cells localized within mucosal tissues that protect against bacterial infections. Upon activation, ILC3s secrete high levels of the cytokine interleukin-22 (IL-22), which is a critical regulator of tissue responses during infection. C. difficile toxin B (TcdB), the major virulence factor, directly activates ILC3s, resulting in high IL-22 levels. We previously reported that polyamines are important in the activation of ILC3s by the innate cytokine interleukin-23 (IL-23) but did not identify a specific mechanism. In this study, we examine how a pathogen impacts a metabolic pathway important for immune cell function and hypothesized that polyamines are important in TcdB-mediated ILC3 activation. We show that TcdB upregulates the polyamine biosynthesis pathway, and the inhibition of the pathway decreases TcdB-mediated ILC3 activation. Two polyamines, putrescine and spermidine, are involved. Spermidine is the key polyamine in the hypusination of eukaryotic initiation factor 5A (eIF5A), and the inhibition of eIF5A reduced ILC3 activation. Thus, there is potential to leverage polyamines in ILC3s to promote activation of ILC3s during C. difficile infection and other bacterial infections where ILC3s serve a protective role.

Modulation of innate lymphoid cells by enteric bacterial pathogens

Innate lymphoid cells (ILCs) are key regulators of tissue homeostasis, inflammation, and immunity to infections. ILCs rapidly respond to environmental cues such as cytokines, microbiota and invading pathogens which regulate their function and phenotype. Even though ILCs are rare cells, they are enriched at barrier surfaces such as the gastrointestinal (GI) tract, and they are often critical to the host's immune response to eliminate pathogens. On the other side of host-pathogen interactions, pathogenic bacteria also have the means to modulate these immune responses. Manipulation or evasion of the immune cells is often to the pathogen's benefit and/or to the detriment of competing microbiota. In some instances, specific bacterial virulence factors or toxins have been implicated in how the pathogen modulates immunity. In this review, we discuss the recent progress made towards understanding the role of non-cytotoxic ILCs during enteric bacterial infections, how these pathogens can modulate the immune response, and the implications these have on developing new therapies to combat infection.

Cx3cr1 controls kidney resident macrophage heterogeneity

Kidney macrophages are comprised of both monocyte-derived and tissue resident populations; however, the heterogeneity of kidney macrophages and factors that regulate their heterogeneity are poorly understood. Herein, we performed single cell RNA sequencing (scRNAseq), fate mapping, and parabiosis to define the cellular heterogeneity of kidney macrophages in healthy mice. Our data indicate that healthy mouse kidneys contain four major subsets of monocytes and two major subsets of kidney resident macrophages (KRM) including a population with enriched Ccr2 expression, suggesting monocyte origin. Surprisingly, fate mapping data using the newly developed Ms4a3Cre Rosa Stopf/f TdT model indicate that less than 50% of Ccr2+ KRM are derived from Ly6chi monocytes. Instead, we find that Ccr2 expression in KRM reflects their spatial distribution as this cell population is almost exclusively found in the kidney cortex. We also identified Cx3cr1 as a gene that governs cortex specific accumulation of Ccr2+ KRM and show that loss of Ccr2+ KRM reduces the severity of cystic kidney disease in a mouse model where cysts are mainly localized to the kidney cortex. Collectively, our data indicate that Cx3cr1 regulates KRM heterogeneity and niche-specific disease progression.

The Polyamine Putrescine Is a Positive Regulator of Group 3 Innate Lymphocyte Activation

Group 3 innate lymphocytes (ILC3s) rapidly respond to invading pathogens or inflammatory signals, which requires shifting cellular metabolic demands. Metabolic adaptations regulating ILC3 function are not completely understood. Polyamines are polycationic metabolites that have diverse roles in cellular functions and in immunity regulate immune cell biology, including Th17 cells. Whether polyamines play a role in ILC3 activation is unknown. In this article, we report that the polyamine synthesis pathway is important for ILC3 activation. IL-23-activated mouse ILC3s upregulate ornithine decarboxylase, the enzyme catalyzing the rate-limiting step of the conversion of ornithine to putrescine in polyamine synthesis, with a subsequent increase in putrescine levels. Inhibition of ornithine decarboxylase via a specific inhibitor, α-difluoromethylornithine, reduced levels of IL-22 produced by steady-state or IL-23-activated ILC3s in a putrescine-dependent manner. Thus, the polyamine putrescine is a positive regulator of ILC3 activation. Our results suggest that polyamines represent a potential target for therapeutic modulation of ILC3 activation during infection or inflammatory disorders.

Group 3 innate lymphocytes (ILC3s) upregulate IL-22 in response to elevated intracellular cAMP levels

Group 3 innate lymphocytes (ILC3s) are important immune cells within mucosal tissues and protect against bacterial infections. They can be activated in response to the innate cytokines IL-23 or IL-1β, which rapidly increases their production of effector molecules that regulate barrier functions. Pathogens can subvert these anti-bacterial effects to evade mucosal defenses to infect the host. Bacillus anthracis, the causative agent of anthrax, produces two major toxins that can modulate the immune response. We have previously shown that lethal toxin downmodulates the function of ILC3s. On the other hand, edema toxin has been shown promote T helper 17 (Th17) cell differentiation, adaptive counterparts of ILC3s, via elevation of cyclic adenosine monophosphate (cAMP). We hypothesized that edema toxin may also modulate ILC3 function. In this study, we show that edema toxin has the opposite effect of lethal toxin; edema toxin directly activates ILC3s independently of innate cytokine stimulation. Treatment of a mouse ILC3-like cell line with edema toxin, a potent adenylate cyclase, upregulated production of the cytokine IL-22, a major effector molecule of ILC3s and a critical factor in maintaining mucosal barriers. Forskolin treatment phenocopied the effect observed with edema toxin and led to an increase in CREB phosphorylation in ILC3s. This observation has potential implications for a role for cAMP signaling in the activation of ILC3s.

IL-22 Binding Protein (IL-22BP) in the Regulation of IL-22 Biology

Cytokines are powerful mediators of inflammation. Consequently, their potency is regulated in many ways to protect the host. Several cytokines, including IL-22, have coordinating binding proteins or soluble receptors that bind to the cytokine, block the interaction with the cellular receptor, and thus prevent cellular signaling. IL-22 is a critical cytokine in the modulation of tissue responses during inflammation and is highly upregulated in many chronic inflammatory disease patients, including those with psoriasis, rheumatoid arthritis, and inflammatory bowel disease (IBD). In healthy individuals, low levels of IL-22 are secreted by immune cells, mainly in the gastrointestinal (GI) tract. However, much of this IL-22 is likely not biologically active due to the high levels of IL-22 binding protein (IL-22BP) produced by intestinal dendritic cells (DCs). IL-22BP is a soluble receptor homolog that binds to IL-22 with greater affinity than the membrane spanning receptor. Much is known regarding the regulation and function of IL-22 in health and disease. However, less is known about IL-22BP. In this review, we will focus on IL-22BP, including its regulation, role in IL-22 biology and inflammation, and promise as a therapeutic. IL-22 can be protective or pathogenic, depending on the context of inflammation. IL-22BP also has divergent roles. Ongoing and forthcoming studies will expand our knowledge of IL-22BP and IL-22 biology, and suggest that IL-22BP holds promise as a way to regulate IL-22 biology in patients with chronic inflammatory disease.

E3 Ubiquitin Ligase Von Hippel-Lindau Protein Promotes Th17 Differentiation

Von Hippel-Lindau (VHL) is an E3 ubiquitin ligase that targets proteins, including HIF-1α, for proteasomal degradation. VHL and HIF regulate the balance between glycolysis and oxidative phosphorylation, which is critical in highly dynamic T cells. HIF-1α positively regulates Th17 differentiation, a complex process in which quiescent naive CD4 T cells undergo transcriptional changes to effector cells, which are commonly dysregulated in autoimmune diseases. The role of VHL in Th17 cells is not known. In this study, we hypothesized VHL negatively regulates Th17 differentiation and deletion of VHL in CD4 T cells would elevate HIF-1α and increase Th17 differentiation. Unexpectedly, we found that VHL promotes Th17 differentiation. Mice deficient in VHL in their T cells were resistant to an autoimmune disease, experimental autoimmune encephalomyelitis, often mediated by Th17 cells. In vitro Th17 differentiation was impaired in VHL-deficient T cells. In the absence of VHL, Th17 cells had decreased activation of STAT3 and SMAD2, suggesting that VHL indirectly or directly regulates these critical signaling molecules. Gene expression analysis revealed that in Th17 cells, VHL regulates many cellular pathways, including genes encoding proteins involved indirectly or directly in the glycolysis pathway. Compared with wild-type, VHL-deficient Th17 cells had elevated glycolysis and glycolytic capacity. Our finding has implications on the design of therapeutics targeting the distinct metabolic needs of T cells to combat chronic inflammatory diseases.

Glucocorticoids Inhibit Group 3 Innate Lymphocyte IL-22 Production

Glucocorticoids (GCs) are commonly prescribed to patients with a variety of inflammatory disorders, including inflammatory bowel disease (IBD). GCs mediate their immunomodulatory effects through many different mechanisms and target multiple signaling pathways. The GC dexamethasone downmodulates innate and adaptive immune cell activation. IBD is the manifestation of a dysregulated immune response involving many different immune cells. Group 3 innate lymphocytes (ILC3s) have critical roles in mucosal inflammation. ILC3s secrete high levels of the cytokine IL-22, promoting epithelial proliferation, antimicrobial peptides, and mucins. In this study, we examined the effects of dexamethasone on IL-22 production by ILC3s. We found that dexamethasone suppressed IL-23-mediated IL-22 production in human and mouse ILC3s. This was mediated in part through dexamethasone modulation of the NF-κB pathway. Inhibition of NF-κB signaling with a small molecule inhibitor also downmodulated IL-23- and IL-1β-mediated IL-22 production in ILC3s. These findings implicate NF-κB as a regulator of IL-22 in ILC3s and likely have repercussions on GC treatment of IBD patients.

IL-22: There Is a Gap in Our Knowledge

IL-22 is a critical cytokine in modulating tissue responses during inflammation. IL-22 is upregulated in many chronic inflammatory diseases, making IL-22 biology a potentially rewarding therapeutic target. However, this is complicated by the dual-natured role of IL-22 in inflammation, as the cytokine can be protective or inflammatory depending on the disease model. Although scientific interest in IL-22 has increased considerably in the past 10 y, there is still much we do not know about the environmental, cellular, and molecular factors that regulate the production and function of this cytokine. A better understanding of IL-22 biology will allow us to develop new or improved therapeutics for treating chronic inflammatory diseases. In this article, I will highlight some of the outstanding questions in IL-22 biology.

IL-22 deficiency increases CD4 T cell responses to mucosal immunization

Mucosal vaccines are a promising platform for combatting infectious diseases for which we still lack effective preventative measures. Optimizing these vaccines to generate the best protective immune responses with the least complicated immunization regimen is imperative. Mucosal barriers are the first line of defense against many pathogens and, as such, we looked to their biology for strategies to improve vaccine delivery. Interleukin-22 (IL-22) is a key cytokine in both healthy and inflamed mucosal tissues. IL-22 promotes epithelial cell proliferation and inhibits apoptosis, upregulates mucin and antimicrobial peptides, all of which promote mucosal barrier integrity. In this study, we find that IL-22 impairs the development of a T cell response during mucosal immunization. Compared to wild-type control mice, IL-22 deficient mice had increased antigen-specific CD4 T cell responses to intrarectal immunization using a protein and cholera toxin adjuvant vaccine. When immunized systemically with the same protein antigen adsorbed to alum, no differences in the CD4 T cell response between wild-type and IL-22 deficient mice were detected. This suggests that transiently inhibiting IL-22 during mucosal vaccination could enhance T cell responses. The broad-applicability of this proposed approach would allow for improvement of many existing mucosal vaccine regimens and have positive implications in the development of more efficacious mucosal vaccines.

Glucocorticoids inhibit group 3 innate lymphocyte IL-22 production

Glucocorticoids (GCs) are commonly prescribed to patients with a variety of inflammatory disorders, including inflammatory bowel disease (IBD). IBD is the manifestation of a dysregulated mucosal immune response, usually against commensal bacteria, involving many different immune cells. GCs mediate their immunomodulatory effects through many different mechanisms and target multiple signaling pathways. The GC dexamethasone down-modulates both innate and adaptive immune cell activation. Group 3 innate lymphocytes (ILC3s) have critical roles in mucosal inflammation. ILC3s secrete high levels of the cytokine IL-22, promoting epithelial proliferation, antimicrobial peptides and mucins. The effects of dexamethasone on T cell function are well described, however, it is not known if dexamethasone modulates the critical function of ILC3s or if dexamethasone regulates the cytokine IL-22. In this study, we examined the effects of dexamethasone on IL-22 production by ILC3s. We found that dexamethasone suppressed IL-23-mediated IL-22 production in homeostatic and activated human and mouse ILC3s. Dexamethasone did not modulate the IL-23 canonical signaling pathway of JAK2/STAT3, but did suppress phosphorylation of IkBa, an important regulator in NF-kB activation. These findings implicate NF-kB as a regulator of IL-22 production in ILC3s and have likely repercussions on GC treatment of IBD patients.

Bacillus anthracis lethal toxin negatively modulates ILC3 function through perturbation of IL-23-mediated MAPK signaling

Bacillus anthracis, the causative agent of anthrax, secretes lethal toxin that down-regulates immune functions. Translocation of B. anthracis across mucosal epithelia is key for its dissemination and pathogenesis. Group 3 innate lymphocytes (ILC3s) are important in mucosal barrier maintenance due to their expression of the cytokine IL-22, a critical regulator of tissue responses and repair during homeostasis and inflammation. We found that B. anthracis lethal toxin perturbed ILC3 function in vitro and in vivo, revealing an unknown IL-23-mediated MAPK signaling pathway. Lethal toxin had no effects on the canonical STAT3-mediated IL-23 signaling pathway. Rather lethal toxin triggered the loss of several MAP2K kinases, which correlated with reduced activation of downstream ERK1/2 and p38, respectively. Inhibition studies showed the importance of MAPK signaling in IL-23-mediated production of IL-22. Our finding that MAPK signaling is required for optimal IL-22 production in ILC3s may lead to new approaches for targeting IL-22 biology.

Hypoxic modulation of hepatocyte responses to the cytokine interleukin-22

The cytokine interleukin-22 (IL-22) is a potent regulator of tissue responses during inflammation. Depending on the context of inflammation, IL-22 can have protective or inflammatory effects on epithelial cells. This dual nature of IL-22 leads us to hypothesize that its activity must be exquisitely regulated to prevent host tissue damage. Environmental factors may act as a cellular cue as to how cells respond to IL-22. Inflammatory environments are characterized by low oxygen and thus we examined whether cells respond differently to IL-22 hypoxia compared with normoxia. In this study, we show that hepatocyte responses to IL-22 stimulation are reduced in hypoxic environments. IL-22 stimulation of hepatocytes incubated in low oxygen led to reduced levels of activated signal transducer and activator of transcription 3 and further downstream effects such as reduced induction of the anti-microbial protein, lipocalin-2. This modulation appears to be independent of the hypoxia-inducible factor-1α signaling pathway. Thus, hypoxia that accompanies chronic inflammation may be a mechanism to regulate the bioactivity of the dual-natured IL-22 cytokine.

Transcription Factor HIF-1α Controls Expression of the Cytokine IL-22 in CD4 T Cells

IL-22 is expressed by activated lymphocytes and is important in modulation of tissue responses during inflammation. The cytokine induces proliferative and antiapoptotic pathways in epithelial cells allowing enhanced cell survival. This can have positive effects, such as in the maintenance of epithelial barriers in the gastrointestinal tract, but also negative effects, such as contributing to colorectal tumorigenesis. Because IL-22 can be dual-natured, we hypothesized that its biological activity should be tightly regulated to limit IL-22 expression to the sites of inflammation. One such environmental cue could be low oxygen, which often accompanies inflammation. We show that in CD4 T cells IL-22 expression is upregulated in hypoxia. The Il22 promoter contains a putative conserved hypoxic response element suggesting that the transcription factor HIF-1α may influence IL-22 expression. Differentiation in the presence of dimethyloxallyl glycine, a stabilizer of HIF-1α at normoxia, increased IL-22 expression. Using HIF-1α-deficient CD4 T cells, we show that hypoxic IL-22 upregulation is dependent on HIF-1α. These findings have implications on the regulation of Il22 gene expression and the presence of the cytokine in different inflammatory environments.

Defective Intestinal Mucin-Type O-Glycosylation Causes Spontaneous Colitis-Associated Cancer in Mice

BACKGROUND & AIMS

Core 1- and core 3-derived mucin-type O-linked oligosaccharides (O-glycans) are major components of the colonic mucus layer. Defective forms of colonic O-glycans, such as the Thomsen-nouveau (Tn) antigen, frequently are observed in patients with ulcerative colitis and colorectal cancer, but it is not clear if they contribute to their pathogenesis. We investigated whether and how impaired O-glycosylation contributes to the development of colitis-associated colorectal cancer using mice lacking intestinal core 1- and core 3-derived O-glycans.

METHODS

We generated mice that lack core 1- and core 3-derived intestinal O-glycans (DKO mice) and analyzed them, along with mice that singly lack intestinal epithelial core 1 O-glycans (IEC C1galt1(-/-) mice) or core 3 O-glycans (C3Gnt(-/-) mice). Intestinal tissues were collected at different time points and analyzed for levels of mucin and Tn antigen, development of colitis, and tumor formation using imaging, quantitative polymerase chain reaction, immunoblot, and enzyme-linked immunosorbent assay techniques. We also used cellular and genetic approaches, as well as intestinal microbiota depletion, to identify inflammatory mediators and pathways that contribute to disease in DKO and wild-type littermates (controls).

RESULTS

Intestinal tissues from DKO mice contained higher levels of Tn antigen and had more severe spontaneous chronic colitis than tissues from IEC C1galt1(-/-) mice, whereas spontaneous colitis was absent in C3GnT(-/-) and control mice. IEC C1galt1(-/-) mice and DKO mice developed spontaneous colorectal tumors, although the onset of tumors in the DKO mice occurred earlier (age, 8-9 months) than that in IEC C1galt1(-/-) mice (15 months old). Antibiotic depletion of the microbiota did not cause loss of Tn antigen but did reduce the development of colitis and cancer formation in DKO mice. Colon tissues from DKO mice, but not control mice, contained active forms of caspase 1 and increased caspase 11, which were reduced after antibiotic administration. Supernatants from colon tissues of DKO mice contained increased levels of interleukin-1β and interleukin-18, compared with those from control mice. Disruption of the caspase 1 and caspase 11 genes in DKO mice (DKO/Casp1/11(-/-) mice) decreased the development of colitis and cancer, characterized by reduced colonic thickening, hyperplasia, inflammatory infiltrate, and tumors compared with DKO mice.

CONCLUSIONS

Impaired expression of O-glycans causes colonic mucus barrier breach and subsequent microbiota-mediated activation of caspase 1-dependent inflammasomes in colonic epithelial cells of mice. These processes could contribute to colitis-associated colon cancer in humans.

The transcription factor HIF1α controls expression of the cytokine IL-22 in CD4 T cells

IL-22 is expressed by activated lymphocytes and is important in modulation of tissue responses during inflammation. The cytokine induces proliferative and anti-apoptotic pathways in epithelial cells allowing enhanced cell survival. This can have positive effects, such as in the maintenance of epithelial barriers in the gastrointestinal tract, but also negative effects, such as contributing to colorectal tumorigenesis. As IL-22 can be dual-natured, we hypothesized that its biological activity should be tightly regulated in order to limit IL-22 expression to the sites of inflammation. One such environmental cue could be low oxygen, which often accompanies inflammation. We show that in CD4 T cells IL-22 expression is upregulated in hypoxia, and this upregulation is dependent on the transcription factor hypoxia-inducible factor 1α (HIF1α). This finding has implications on the regulation of Il22 gene expression and the cytokine’s presence in different inflammatory environments.

IL-22 deficiency alters colonic microbiota to be transmissible and colitogenic

IL-22 is a good candidate to play a critical role in regulating gut microbiota because it is an important inducer of antimicrobial peptides and mucins in the gut. However, whether IL-22 participates in immune homeostasis by way of modulating gut microbiota remains to be elucidated. In this study, we find, through 16S rRNA gene-pyrosequencing analysis, that healthy IL-22-deficient mice had altered colonic microbiota, notably with decreased abundance of some genera, including Lactobacillus, and increased levels of others. Mice harboring this altered microbiota exhibited more severe disease during experimentally induced colitis. Interestingly, this altered gut microbiota can be transmitted to cohoused wild-type animals along with the increased susceptibility to this colitis, indicating an important role for IL-22 in shaping the homeostatic balance between immunity and colonic microbiota for host health.

IL-22BP is regulated by the inflammasome and modulates tumorigenesis in the intestine

Chronic mucosal inflammation and tissue damage predisposes patients to the development of colorectal cancer. This association could be explained by the hypothesis that the same factors and pathways important for wound healing also promote tumorigenesis. A sensor of tissue damage should induce these factors to promote tissue repair and regulate their action to prevent development of cancer. Interleukin 22 (IL-22), a cytokine of the IL-10 superfamily, has an important role in colonic epithelial cell repair, and its levels are increased in the blood and intestine of inflammatory bowel disease patients. This cytokine can be neutralized by the soluble IL-22 receptor, known as the IL-22 binding protein (IL-22BP, also known as IL22RA2); however, the significance of endogenous IL-22BP in vivo and the pathways that regulate this receptor are unknown. Here we describe that IL-22BP has a crucial role in controlling tumorigenesis and epithelial cell proliferation in the colon. IL-22BP is highly expressed by dendritic cells in the colon in steady-state conditions. Sensing of intestinal tissue damage via the NLRP3 or NLRP6 inflammasomes led to an IL-18-dependent downregulation of IL-22BP, thereby increasing the ratio of IL-22/IL-22BP. IL-22, which is induced during intestinal tissue damage, exerted protective properties during the peak of damage, but promoted tumour development if uncontrolled during the recovery phase. Thus, the IL-22-IL-22BP axis critically regulates intestinal tissue repair and tumorigenesis in the colon.

IL-22 signaling contributes to West Nile encephalitis pathogenesis

The Th17 cytokine, IL-22, regulates host immune responses to extracellular pathogens. Whether IL-22 plays a role in viral infection, however, is poorly understood. We report here that Il22(-/-) mice were more resistant to lethal West Nile virus (WNV) encephalitis, but had similar viral loads in the periphery compared to wild type (WT) mice. Viral loads, leukocyte infiltrates, proinflammatory cytokines and apoptotic cells in the central nervous system (CNS) of Il22(-/-) mice were also strikingly reduced. Further examination showed that Cxcr2, a chemokine receptor that plays a non-redundant role in mediating neutrophil migration, was significantly reduced in Il22(-/-) compared to WT leukocytes. Expression of Cxcr2 ligands, cxcl1 and cxcl5, was lower in Il22(-/-) brains than wild type mice. Correspondingly, neutrophil migration from the blood into the brain was attenuated following lethal WNV infection of Il22(-/-) mice. Our results suggest that IL-22 signaling exacerbates lethal WNV encephalitis likely by promoting WNV neuroinvasion.

NLRP10 is a NOD-like receptor essential to initiate adaptive immunity by dendritic cells

NLRs (nucleotide-binding domain leucine-rich repeat containing receptors; NOD-like receptors) are a class of pattern recognition receptor (PRR) that respond to host perturbation from either infectious agents or cellular stress^1,2. The function of most NLR family members has not been characterized and their role in instructing adaptive immune responses remains unclear ^2,3. NLRP10 (also known as PYNOD, NALP10, PAN5 and NOD8) is the only NLR lacking the putative ligand binding leucine rich repeat domain, and has been postulated to be a negative regulator of other NLR members including NLRP3^4–6. We did not find evidence that NLRP10 functions through an inflammasome to regulate caspase-1 activity nor that it regulates other inflammasomes. Instead, Nlrp10^−/− mice had a profound defect in helper T cell-driven immune responses to a diverse array of adjuvants including lipopolysaccharide (LPS), aluminium hydroxide (alum) and complete Freund’s adjuvant (CFA). Adaptive immunity was impaired in the absence of NLRP10 due to a dendritic cell (DC) intrinsic defect in emigration from inflamed tissues while upregulation of DC costimulatory molecules and chemotaxis to CCR7-dependent and independent ligands remained intact. The loss of antigen transport to the draining LN by this migratory DC subset resulted in an almost absolute loss in naïve CD4⁺ T cell priming, highlighting the critical link between diverse innate immune stimulation, NLRP10 activity and the immune function of mature DCs.

Interleukin 23 production by intestinal CD103(+)CD11b(+) dendritic cells in response to bacterial flagellin enhances mucosal innate immune defense

Microbial penetration of the intestinal epithelial barrier triggers inflammatory responses that include induction of the bactericidal C-type lectin RegIIIγ. Systemic administration of flagellin, a bacterial protein that stimulates Toll-like receptor 5 (TLR5), induces epithelial expression of RegIIIγ and protects mice from intestinal colonization with antibiotic-resistant bacteria. Flagellin-induced RegIIIγ expression is IL-22 dependent, but how TLR signaling leads to IL-22 expression is incompletely defined. By using conditional depletion of lamina propria dendritic cell (LPDC) subsets, we demonstrated that CD103(+)CD11b(+) LPDCs, but not monocyte-derived CD103(-)CD11b(+) LPDCs, expressed high amounts of IL-23 after bacterial flagellin administration and drove IL-22-dependent RegIIIγ production. Maximal expression of IL-23 subunits IL-23p19 and IL-12p40 occurred within 60 min of exposure to flagellin. IL-23 subsequently induced a burst of IL-22 followed by sustained RegIIIγ expression. Thus, CD103(+)CD11b(+) LPDCs, in addition to promoting long-term tolerance to ingested antigens, also rapidly produce IL-23 in response to detection of flagellin in the lamina propria.

Recent advances in IL-22 biology

Several cell types, in particular epithelial cells, express the receptor for the cytokine IL-22 and upon its recognition produce molecules that are active both locally and systemically. Many different types of lymphocyte secrete IL-22. T(h)17 cells produce IL-22 although the optimal conditions for secretion of IL-17 or IL-22 by T(h)17 cells differ, as do the transcription factors involved. Aryl hydrocarbon receptor is required for IL-22 production by T(h)17, T(h)22 and γδ T cells. T(h)22 cells produce IL-22 in response to IL-6 and tumor necrosis factor α (TNF-α), particularly in the skin, whereas γδ T cells produce IL-22 in response to IL-23, particularly in the lung. NK cells produce IL-22 in response to IL-12 and IL-18 or IL-23. Retinoic acid-related orphan receptorγt-positive innate lymphoid cells, including lymphoid tissue inducer (LTi) and LTi-like cells express IL-22 with IL-23 again enhancing expression. IL-22 is known to be expressed in many chronic inflammatory conditions, including psoriasis and rheumatoid arthritis, and its up-regulation often correlates with disease activity. IL-22 is known to be protective in the gastrointestinal tract in inflammatory bowel disease but may mediate either harmful or helpful inflammatory responses in different models of intestinal infection. Finally, IL-22 may also play an important role in tissue repair.

Memory/effector (CD45RB(lo)) CD4 T cells are controlled directly by IL-10 and cause IL-22-dependent intestinal pathology

The role of direct IL-10 signaling in different T cell subsets is not well understood. To address this, we generated transgenic mice expressing a dominant-negative IL-10 receptor specifically in T cells (CD4dnIL-10Rα). We found that Foxp3-depleted CD45RB(lo) (regulatory T cell [T(reg) cell]-depleted CD45RB(lo)) but not CD45RB(hi) CD4(+) T cells are controlled directly by IL-10 upon transfer into Rag1 knockout (KO) mice. Furthermore, the colitis induced by transfer of T(reg) cell-depleted CD45RB(lo) CD4(+) T cells into Rag1 KO mice was characterized by reduced Th1 and increased Th17 cytokine messenger RNA levels in the colon as compared with the colitis induced by transfer of CD45RB(hi) T cells. In contrast to the CD45RB(hi) transfer colitis model, in which IL-22 is protective, we found that T cell-derived IL-22 was pathogenic upon transfer of T(reg) cell-depleted CD45RB(lo) T cells into Rag1 KO mice. Our results highlight characteristic differences between colitis induced by naive (CD45RB(hi)) and memory/effector (T(reg) cell-depleted CD45RB(lo)) cells and different ways that IL-22 impacts inflammatory bowel disease.

Unraveling the genetics of autoimmunity

The chronic autoimmune diseases include multiple complex genetic disorders. Recently, genome-wide association studies (GWAS) have identified a large number of major loci, with many associations shared between various autoimmune diseases. These associations highlight key roles for lymphocyte activation and prioritize specific cytokine pathways and mechanisms of host-microbe recognition. Despite success in identifying loci, comprehensive models of disease pathogenesis are currently lacking. Future efforts comparing association patterns between autoimmune diseases may be particularly illustrative. New genomic technologies applied to classic genetic studies involving twins, early onset cases, and phenotypic extremes may provide key insights into developmental and gene-environment interactions in autoimmunity.

Bacterial flagellin stimulates Toll-like receptor 5-dependent defense against vancomycin-resistant Enterococcus infection

Treatment of vancomycin-resistant Enterococcus (VRE) infections is limited by the paucity of effective antibiotics. Administration of broad-spectrum antibiotics promotes VRE colonization by down-regulating homeostatic innate immune defenses. Intestinal epithelial cells and Paneth cells express antimicrobial factors on direct or indirect stimulation of the Toll-like receptor (TLR)-myeloid differentiation factor 88-mediated pathway by microbe-derived molecules. Here, we demonstrate that the TLR5 agonist flagellin restores antibiotic-impaired innate immune defenses and restricts colonization with VRE. Flagellin stimulates the expression of RegIIIgamma, a secreted C-type lectin that kills gram-positive bacteria, including VRE. Systemic administration of flagellin induces RegIIIgamma expression in intestinal epithelial cells and Paneth cells along the entire length of the small intestine. Induction of RegIIIgamma requires TLR5 expression in hematopoietic cells and is dependent on interleukin 22 expression. Systemic administration of flagellin to antibiotic-treated mice dramatically reduces VRE colonization. By enhancing mucosal resistance to multidrug-resistant organisms, flagellin administration may provide a clinically useful approach to prevent infections in patients treated with broad-spectrum antibiotics.

RasGRP1 regulates antigen-induced developmental programming by naive CD8 T cells

Ag encounter by naive CD8 T cells initiates a developmental program consisting of cellular proliferation, changes in gene expression, and the formation of effector and memory T cells. The strength and duration of TCR signaling are known to be important parameters regulating the differentiation of naive CD8 T cells, although the molecular signals arbitrating these processes remain poorly defined. The Ras-guanyl nucleotide exchange factor RasGRP1 has been shown to transduce TCR-mediated signals critically required for the maturation of developing thymocytes. To elucidate the role of RasGRP1 in CD8 T cell differentiation, in vitro and in vivo experiments were performed with 2C TCR transgenic CD8 T cells lacking RasGRP1. In this study, we report that RasGRP1 regulates the threshold of T cell activation and Ag-induced expansion, at least in part, through the regulation of IL-2 production. Moreover, RasGRP1(-/-) 2C CD8 T cells exhibit an anergic phenotype in response to cognate Ag stimulation that is partially reversible upon the addition of exogenous IL-2. By contrast, the capacity of IL-2/IL-2R interactions to mediate Ras activation and CD8 T cell expansion and differentiation appears to be largely RasGRP1-independent. Collectively, our results demonstrate that RasGRP1 plays a selective role in T cell signaling, controlling the initiation and duration of CD8 T cell immune responses.

Anti-inflammatory and pro-inflammatory roles of TGF-beta, IL-10, and IL-22 in immunity and autoimmunity

Cytokines play a major role in maintaining lymphocyte homeostasis under both steady-state and inflammatory conditions. Unregulated lymphocytes in steady-state conditions can lead to autoimmunity, whereas during inflammation they can cause excessive tissue damage. Regulatory cytokines function in combination with other environmental signals to properly modulate the function and the extent of lymphocyte activation. Many recent studies have highlighted the importance of regulatory cytokines in controlling the differentiation and function of lymphocytes under steady-state and inflammatory conditions, as well as minimizing tissue damage.

CD4 T-cell differentiation and inflammatory bowel disease

Differentiation of naïve T cells leads to the generation of T-cell subsets, each possessing distinct cytokine expression profiles for serving different immune functions. Through the activation of separate signaling pathways, this process results in both differentiated helper T (Th) cells, termed Th1, Th2 and Th17, and induced regulatory T cells, which suppress Th cells. These different cells are important for combating infectious diseases and cancers; however, when aberrant, they can be responsible for chronic inflammatory diseases. One such disease is inflammatory bowel disease (IBD), in which each T-cell subset can have a role in disease. New studies highlight the importance of the recently identified Th17 subset in IBD. Therapeutics targeting these aberrant Th responses are already under development and hold promise for treating IBD and other chronic inflammatory diseases.

Innate and adaptive interleukin-22 protects mice from inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disease thought to be mediated by dysfunctional innate and/or adaptive immunity. This aberrant immune response leads to the secretion of harmful cytokines that destroy the epithelium of the gastrointestinal tract and thus cause further inflammation. Interleukin-22 (IL-22) is a T helper 17 (Th17) T cell-associated cytokine that is bifunctional in that it has both proinflammatory and protective effects on tissues depending on the inflammatory context. We show herein that IL-22 protected mice from IBD. Interestingly, not only was this protection mediated by CD4+ T cells, but IL-22-expressing natural killer (NK) cells also conferred protection. In addition, IL-22 expression was differentially regulated between NK cell subsets. Thus, both the innate and adaptive immune responses have developed protective mechanisms to counteract the damaging effects of inflammation on tissues.

IL-22 and inflammation: leukin' through a glass onion

IL-22 is a Th17 T-cell-associated cytokine that is highly expressed during chronic inflammation. IL-22 receptor expression is absent on immune cells, but is instead restricted to the tissues, providing signaling directionality from the immune system to the tissues. Through Stat3 signaling, IL-22 induces a variety of proliferative, anti-apoptotic, and anti-microbial pathways. IL-22 is bi-functional with both pro-inflammatory and protective effects on tissues depending on the inflammatory context. The cytokine plays a role both in the host response against extracellular pathogens and in the inflammation associated with autoimmune diseases. Therapeutics targeting IL-22 therefore may have promise for treating various chronic inflammatory diseases.

Chronic immunodeficiency in mice lacking RasGRP1 results in CD4 T cell immune activation and exhaustion

The Ras-guanyl nucleotide exchange factor RasGRP1 is an important link between TCR-mediated signaling and the activation of Ras and its downstream effectors. RasGRP1 is especially critical for the survival and differentiation of developing thymocytes whereas negative selection of thymocytes bearing an autoreactive TCR appears to be RasGRP1 independent. Despite apparently normal central tolerance, RasGRP1(-/-) mice spontaneously acquire an acutely activated and proliferating CD4 T cell population that exhibits characteristics of T cell exhaustion, including strong expression of programmed cell death-1. To elucidate the basis for RasGRP1(-/-) CD4 T cell immune activation, we initiated a series of adoptive transfer experiments. Remarkably, the copious amounts of cytokines and self-Ags present in hosts made lymphopenic through irradiation failed to induce the majority of RasGRP1(-/-) CD4 T cells to enter cell cycle. However, their infusion into either congenitally T cell- or T/B cell-deficient recipients resulted in robust proliferation and L-selectin down-regulation. These findings imply that the activation and proliferation of RasGRP1(-/-) CD4 T cells may be dependent on their residence in a chronically immunocompromised environment. Accordingly, bacterial and viral challenge experiments revealed that RasGRP1(-/-) mice possess a weakened immune system, exhibiting a T cell-autonomous defect in generating pathogen-specific T cells and delayed pathogen clearance. Collectively, our study suggests that chronic T cell immunodeficiency in RasGRP1(-/-) mice may be responsible for CD4 T cell activation, proliferation, and exhaustion.

Interleukin-22 but not interleukin-17 provides protection to hepatocytes during acute liver inflammation

The cytokine interleukin-22 (IL-22) is primarily expressed by T helper 17 (Th17) CD4(+) T cells and is highly upregulated during chronic inflammatory diseases. IL-22 receptor expression is absent on immune cells, but is instead restricted to the tissues, providing signaling directionality from the immune system to the tissues. However, the role of IL-22 in inflammatory responses has been confounded by data suggesting both pro- and anti-inflammatory functions. Herein, we provide evidence that during inflammation, IL-22 played a protective role in preventing tissue injury. Hepatocytes from mice deficient in IL-22 were highly sensitive to the detrimental immune response associated with hepatitis. Additionally, IL-22-expressing Th17 cells provided protection during hepatitis in IL-22-deficient mice. On the other hand, interleukin-17 (IL-17), which is coexpressed with IL-22 and can induce similar cellular responses, had no observable role in liver inflammation. Our data suggest that IL-22 serves as a protective molecule to counteract the destructive nature of the immune response to limit tissue damage.

Innate and adaptive immune responses to Listeria monocytogenes: a short overview

The Gram-positive facultative intracellular bacterium Listeria monocytogenes is a model pathogen for elucidating important mechanisms of the immune response. Infection of mice with a sub-lethal dose of bacteria generates highly reproducible innate and adaptive immune responses, resulting in clearance of the bacteria and resistance to subsequent L. monocytogenes infection. Both the innate and adaptive immune systems are crucial to the recognition and elimination of this pathogen from the host.

Innate and adaptive immune responses to Listeria monocytogenes: a short overview

The Gram-positive facultative intracellular bacterium Listeria monocytogenes is a model pathogen for elucidating important mechanisms of the immune response. Infection of mice with a sub-lethal dose of bacteria generates highly reproducible innate and adaptive immune responses, resulting in clearance of the bacteria and resistance to subsequent L. monocytogenes infection. Both the innate and adaptive immune systems are crucial to the recognition and elimination of this pathogen from the host.

Characterization of Listeria monocytogenes expressing anthrolysin O and phosphatidylinositol-specific phospholipase C from Bacillus anthracis

Two virulence factors of Listeria monocytogenes, listeriolysin O (LLO) and phosphatidylinositol-specific phospholipase C (PI-PLC), mediate escape of this pathogen from the phagocytic vacuole of macrophages, thereby allowing the bacterium access to the host cell cytosol for growth and spread to neighboring cells. We characterized their orthologs from Bacillus anthracis by expressing them in L. monocytogenes and characterizing their contribution to bacterial intracellular growth and cell-to-cell spread. We generated a series of L. monocytogenes strains expressing B. anthracis anthrolysin O (ALO) and PI-PLC in place of LLO and L. monocytogenes PI-PLC, respectively. We found that ALO was active at both acidic and neutral pH and could functionally replace LLO in mediating escape from a primary vacuole; however, ALO exerted a toxic effect on the host cell by damaging the plasma membrane. B. anthracis PI-PLC, unlike the L. monocytogenes ortholog, had high activity on glycosylphosphatidylinositol-anchored proteins. L. monocytogenes expressing B. anthracis PI-PLC showed significantly decreased efficiencies of escape from a phagosome and in cell-to-cell spread. We further compared the level of cytotoxicity to host cells by using mutant strains expressing ALO in combination either with L. monocytogenes PI-PLC or with B. anthracis PI-PLC. The results demonstrated that the mutant strain expressing the combination of ALO and B. anthracis PI-PLC caused less damage to host cells than the strain expressing ALO and L. monocytogenes PI-PLC. The present study indicates that LLO and L. monocytogenes PI-PLC has adapted for L. monocytogenes intracellular growth and virulence and suggests that ALO and B. anthracis PI-PLC may have a role in B. anthracis pathogenesis.

Listeria monocytogenes phosphatidylinositol-specific phospholipase C has evolved for virulence by greatly reduced activity on GPI anchors

Listeria monocytogenes phosphatidylinositol-specific phospholipase C (PI-PLC) plays a critical role in escape of this human pathogen from host cell vacuoles. Unlike classical bacterial PI-PLCs, the L. monocytogenes enzyme has very weak activity on glycosylphosphatidylinositol (GPI)-anchored proteins. Previous crystal structure analysis has revealed that a small beta-strand (Vb) is present in Bacillus cereus PI-PLC and is absent in the enzyme from L. monocytogenes. This Vb beta-strand in B. cereus PI-PLC forms contacts with the glycan linker of GPI anchors, which presumably increases its activity on GPI-anchored proteins. In this study, we show that, of all known bacterial PI-PLCs, those from listeriae are the only ones that lack the beta-strand. Expression by L. monocytogenes of B. cereus PI-PLC, which has strong activity on GPI-anchored proteins, inhibited bacterial escape from a vacuole and cell-to-cell spread, resulting in greatly reduced virulence in mice. Deletion of the Vb beta-strand from B. cereus PI-PLC abolished its ability to cleave GPI-anchored proteins, decreased its inhibitory effects, and increased its virulence in mice. These results strongly suggest that L. monocytogenes PI-PLC has evolved as an important determinant of L. monocytogenes pathogenesis by absence of the Vb beta-strand, thus leading to greatly reduced activity on GPI-anchored proteins.

Sugar-coated regulation of T cells

The human gut is host to hundreds of different species of commensal bacteria that live in peaceful partnership with the host immune system. These commensal bacteria are far from neutral bystanders as they are intimately involved in the development of the immune system. Reporting in this issue of Cell, Kasper and colleagues (Mazmanian et al., 2005) reveal that a bacterial polysaccharide, PSA, produced by the commensal bacterium Bacteroides fragilis directs development of the immune system of the mouse host.

Phosphatidylinositol-Specific Phospholipase C ofBacillus anthracisDown-Modulates the Immune Response

Phosphatidylinositol-specific phospholipases (PI-PLCs) are virulence factors produced by many pathogenic bacteria, including Bacillus anthracis and Listeria monocytogenes. Bacillus PI-PLC differs from Listeria PI-PLC in that it has strong activity for cleaving GPI-anchored proteins. Treatment of murine DCs with Bacillus, but not Listeria, PI-PLC inhibited dendritic cell (DC) activation by TLR ligands. Infection of mice with Listeria expressing B. anthracis PI-PLC resulted in a reduced Ag-specific CD4 T cell response. These data indicate that B. anthracis PI-PLC down-modulates DC function and T cell responses, possibly by cleaving GPI-anchored proteins important for TLR-mediated DC activation.

Activation of antigen-specific CD8 T cells results in minimal killing of bystander bacteria

Memory CD8 T cells play a critical role in protective immunity against intracellular pathogens. In addition to their ability to specifically recognize and lyse infected targets, activated CD8 T cells secrete cytokines that induce phagocytic cells to engulf and kill bacterial pathogens. In this study, we asked whether activation of Ag-specific CD8 T cells results in nonspecific killing of bystander bacteria during a mixed infection. Mice with epitope-specific memory CD8 T cells were coinfected with two isogenic strains of recombinant Listeria monocytogenes that differ in the cognate epitope. Recall responses by epitope-specific CD8 T cells rapidly inhibited the growth of epitope-bearing bacteria, impeding the course of infection within 6 h after challenge. This rapid inhibition was highly specific and did not affect the growth of coinfecting bacteria without the epitope. CTL recall did not enhance activation of innate immune cells, as evidenced by the absence of inducible NO synthase production in infectious foci. Our observations demonstrate the remarkable specificity of the bactericidal mechanisms of CTL and reveal the possibility for escape mutants to prevail in the hostile environment of a specific immune response. This implication has a bearing on subunit vaccine design strategies and understanding failure of immunization against bacterial infection.

Listeria monocytogenes virulence proteins induce surface expression of Fas ligand on T lymphocytes

Virulence factors secreted by Listeria monocytogenes are known to interfere with host cellular signalling pathways. We investigated whether L. monocytogenes modulates T-cell receptor signalling by examining surface expression of proteins known to be upregulated on activated T cells. In vitro culture of murine splenocytes with L. monocytogenes resulted in a specific and dose-dependent upregulation of Fas ligand (FasL). Induction of FasL expression was also observed for pathogenic Listeria ivanovii but not for non-pathogenic Listeria innocua, indicating involvement of Listeria virulence protein(s). Examination of L. monocytogenes strains deficient in different virulence genes demonstrated that FasL upregulation was dependent on the expression of two secreted proteins: listeriolysin O (LLO) and phosphatidylcholine-preferring phospholipase C (PC-PLC). Treatment of cells with purified proteins demonstrated that LLO was sufficient for inducing FasL, while PC-PLC synergized with LLO for the induction of FasL expression. FasL-expressing cells induced by L. monocytogenes were capable of killing Fas-expressing target cells. Furthermore, L. monocytogenes infection results in upregulation of FasL on T cells in mice. These results describe a novel function for LLO and PC-PLC and suggest that L. monocytogenes may use these virulence factors to modulate the host immune response.

Nonsecreted bacterial proteins induce recall CD8 T cell responses but do not serve as protective antigens

Secreted or nonsecreted Ag expressed by recombinant Listeria monocytogenes can prime CD8 T cells. However, Ag-specific memory CD8 T cells confer protection against bacteria secreting Ag, but not against bacteria expressing the nonsecreted form of the same Ag. This dichotomy may be explained by a long-standing hypothesis that nonsecreted Ags are less effective than secreted Ags at inducing a protective immune response at the onset of infection. We tested this hypothesis by examining whether these two different forms of Ag induce different primary and secondary CD8 T cell responses. The primary responses to secreted and nonsecreted Ags expanded and contracted almost synchronously, although the responses to nonsecreted Ags were of lower magnitude. These results demonstrate that the kinetics of the CD8 T cell response are similar regardless of whether Ag is accessible to the endogenous MHC class I pathway or can only be presented through cross-presentation. No differences were detected in the CD8 T cell recall response to L. monocytogenes expressing secreted or nonsecreted Ags. Nonsecreted Ags are as effective as secreted Ags at the induction of a rapid recall response by memory CD8 T cells. Thus, the inability of nonsecreted bacterial proteins to serve as protective Ags cannot be attributed to a defective CD8 T cell response.

Cutting edge: CD4 and CD8 T cells are intrinsically different in their proliferative responses

In this study, we compared the proliferation and differentiation of Ag-specific CD4 and CD8 T cells following Listeria infection. Our results show that CD4 T cells responding to infection divide a limited number of times, with progeny exhibiting proliferative arrest in early divisions. Even with increased infectious doses, CD4 T cells display this restricted proliferative pattern and are not driven to undergo extensive clonal expansion. This is in striking contrast to CD8 T cells, which undergo extensive proliferation in response to infection. These differences are also evident when CD4 and CD8 T cells receive uniform anti-CD3 stimulation in vitro. Together, these results suggest that CD4 and CD8 T cells are programmed to undergo limited and extensive proliferation, respectively, to suit their function as regulator and effector cells.

Changes in availability of oxygen accentuate differences in capsular polysaccharide expression by phenotypic variants and clinical isolates of Streptococcus pneumoniae

Most isolates of Streptococcus pneumoniae are mixed populations of transparent (T) and opaque (O) colony phenotypes. Differences in the production of capsular polysaccharide (CPS) between O and T variants were accentuated by changes in the environmental concentration of oxygen. O variants demonstrated a 5.2- to 10.6-fold increase in amounts of CPS under anaerobic compared to atmospheric growth conditions, while CPS production remained low under all conditions for T variants. Increased amounts of CPS in O compared to T pneumococci were associated with increased expression of cps-encoded proteins. The inhibitory effect of oxygen on expression of CPS in O variants correlated with decreased tyrosine phosphorylation of CpsD, a tyrosine kinase and regulator of CPS synthesis. Modulation of CpsD expression and its activity by tyrosine phosphorylation may allow the pneumococcus to adapt to the requirements of both colonization, where decreased CPS allows for adherence, and bacteremia, where increased CPS may be required to escape from opsonic clearance. In patients with invasive infection, paired isolates from the same patient were shown to have predominantly a T colony phenotype without phosphotyrosine on CpsD when cultured from the nasopharynx, and an O phenotype that phosphorylates CpsD in response to oxygen when cultured from the blood. Differences in the availability of oxygen, therefore, may be a key factor in allowing for the selection of distinct phenotypes in these two host environments.

Synergy in a medicinal plant: antimicrobial action of berberine potentiated by 5'-methoxyhydnocarpin, a multidrug pump inhibitor

Multidrug resistance pumps (MDRs) protect microbial cells from both synthetic and natural antimicrobials. Amphipathic cations are preferred substrates of MDRs. Berberine alkaloids, which are cationic antimicrobials produced by a variety of plants, are readily extruded by MDRs. Several Berberis medicinal plants producing berberine were found also to synthesize an inhibitor of the NorA MDR pump of a human pathogen Staphylococcus aureus. The inhibitor was identified as 5'-methoxyhydnocarpin (5'-MHC), previously reported as a minor component of chaulmoogra oil, a traditional therapy for leprosy. 5'-MHC is an amphipathic weak acid and is distinctly different from the cationic substrates of NorA. 5'-MHC had no antimicrobial activity alone but strongly potentiated the action of berberine and other NorA substrates against S. aureus. MDR-dependent efflux of ethidium bromide and berberine from S. aureus cells was completely inhibited by 5'-MHC. The level of accumulation of berberine in the cells was increased strongly in the presence of 5'-MHC, indicating that this plant compound effectively disabled the bacterial resistance mechanism against the berberine antimicrobial.